Electromechanical control system



Aug. 4, 1942. K. SINGER 2,292,166

ELECTRO*MBCHANICAL CONTROL SYSTEM Filed June 20, 1959 GAL VANO/W5 n P/Q /gff AMPL /F/Ee A Mp2 /F/EZ c @En 5,4 cK

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Patented Aug. 4, 1942` 2,292,166 ELEC'raoMEcnANIcAL coN'raoL SYSTEM Kurt Singer, Burbank, Calif., assignor to Radio -Corporation of America, a corporation of Dela- Ware Application June 20, 1939, Serial No. 280,031

6 Claims.

This invention relates to electrical control circuits and particularly to a feedback circuit arrangement for neutralizing undesired resonance characteristics in an electrical element.

, In the art of sound recording, a well known method of and system for modulating a light beam being impressed upon a sound track area of a film is by the use of a galvanometer having l a moving armature which tilts a mirror for deilecting the beam transversely to the film. The amature of the galvanometer is actuated by a magnetic field produced by a coil connected to the output of the speech or audio-frequency amplifying equipment which, in turn, may be connected to a microphone or other translating device, such as a phonograph pickup or the like. The armature of the galvanometer is usually tensioned, which produces a mechanical resonance at one or more frequencies, while there may frequently be an electrical resonant point somewhere in the system.

In view of these conditions, the deflection of the mirror is not uniform for the same current input from the speech amplifying equipment at varying frequencies, thus causing distortion of the sound beinggimpressedupon the film. For optimum operation, the frequency-.response characteristic of the galvanometer should be flat over the entire frequency range.

The present invention minimizes the effect of any distortion of the galvanometer frequency characteristic and thereby permits of greater tolerances in the manufacture of the galvanometer and the electrical circuit used therewith. In brief, the operation of the invention is that voltages in accordance with the frequency-response characteristic of thegalvanometer are generated, and these voltages are amplified, reversed in phase and fed back into the speech input amplifying system. Since the feedback voltages are in phase opposition to the original voltages, a cancellation or neutralization will `'take place in the electrical circuit, thereby producing the `flat frequency characteristic desired.

The generation of the feedback voltages is accomplished by interepting a portion of the light beam of the sound recording system, these light waves being impressed upon a photoelectric` cell. By this arrangement, any galvanometer having` any particular type of frequency-response characteristic may have its response corrected re- Another object of the invention is to maintain a constant relationship between frequency input and the response of a moving element actuated by electrical currents.

A further object of the invention is to reintroduce into an amplifying system voltages directly proportional to movements caused by electrical currents originating from this 'amplifying system.

A further object ofthe invention is to provide a feedback circuit for a soundrecording galvanometer wherein the feedback voltages are obtained by intercepting light beam variations produced by the galvanometer.

Although the novel features which are believed to be characteristic of this invention are pointed out with particularity in the claims appended herewith, the manner'of its organization and the mode of its operation will be better understood by referring to the following description read in conjunction with the accompanying ,drawing forming a part thereof, in which Figure 1 is a `diagrammatic arrangement of a sound recording system embodying the invention; and

Figure 2 is a graph showing characteristics of the system and the manner of its operation.

Referring now to Fig. 1, amicrophone l feeds an audiofrequency or speech input amplifier 6.

gardless of the construction of the galvanometer` The output of the amplifier 0 is .fed into a `galvanometer t of any well known type having a mirror III which tilts or vibrates in accordance with the amplitude and frequency of the sound waves to be recorded. The light source of the apparatus comprises a lamp represented by a filament l2, the light therefrom being projected through a lens and mask unit Il to the mirror I0 of the galvanometer. Reected iight from the mirror is projected ic a film I5 through an optical slit assembly I6.` A well known type of sound recording system so represented is disclosed and claimed in U. S. Patents 2,102,776; 2,102,777 and 2,102,778. l

To obtain the voltages produced by the actual response of the galvanometer as represented by the movements of the mirror I0, a reflector I8 is placed so as to intercept light between the mirror I0 and the assembly". 'I'his reflector is so positioned that it does not interfere with the portion of the beam being projected to the film i5 but does intercept the outer portions of the beam which varyeither in intensity or area in the same relationship as the beam being impressed on the nlm.

The light renected `from mirror Il passes through a slit I9 and by means of a lens 20 is projected asa variable miensitybeam upon the cathode of aphotoeleotric cell 22. Thus the light reaching the cellv 22 will vary in accordance with the vibrations of the mirror i and will create at the input to a feedback amplifier ziivoltages' -4 directly proportional to the movements of the mirror i0. 'Ifheoutput ofthe feedback ampliner' passes through a pair of series connected phase.

and amplitude adjusting circuits 25 and zu, such asillustrated in il. Si. Patent Zlltld of August arcaico Y eine element, a light desee-ter actuated by sais clement, means for impressing currentsof vary f ing amplitudes and frequencies on said device for 2u, 1935. The arms of the bridge comprise im i vanometer frequency characteristic without theY feedbackicircuitV i8-28.

It will `be noted that curve A is not a straightline as shown by curve B, curve B being im` idealvcharacteristic. It will also be noted from curve A that the frequency response is slightly higher in the lower frequency region, lower inthe middle frequency region and particularly high in the upper frequency region. I'his last peak is caused by mechanical resonance. However, with the particular feedback circuit of applicants invention, it is not vonly possible to eliminate the high resonance peak occurring at around nine thousand cycles, but also the smaller variations in response. I

On the graph shown in Fig. 2 is a dotted curve C. Since the response of the galvanometer is shown by curve A and the voltage generated in the cell 22 vis directly proportional to the movef ments of the mirror Ill, a voltage'proportional to curve A will be impressed upon the amplifier 24 from cell 22. Since this voltage is reversed in phase and fed back to the input of 'amplifier l.

` its effect upon the input to the galvanometer is similar to a voltage input having a frequency characteristic as shown by curve C. Thus, curve actuating said movable element, means for inters cepting light deectedby said light defiector,

lmeans for translating variations of light caused by said defiector into 'voltages corresponding therewith. and means for reintroducing said vult-= ages into said current impressing means.

2. A resonance correcting system in accordance with. claim l in which said last-mentioned means includes an amplifier and phase and frequency response adjusting networks.

3. In a sound recording system the combination of means for producing a. light beam, means for projecting said beam to aV film, amirror for vibrating rsaid beam with respect to'said film,

a movable' armature-,for actuating saidmirror, an-

n'ductance surrounding ,said armature, means for impressing currents having varying amplitudes and frequencies on said inductance for moving said armature and said mirror, means intermediate said mirror and-said film for translating light deflections caused by said mirror into corresponding voltages, and means for reintroducing said voltages into said current impressing means, said last-mentioned means comprising a. network of phase and frequency characteristic adjusting ele.. ments. n

4. The method of neutralizing mechanical and electrical resonance peaks in an velectro-:riechenical device for modulating light comprising generating vwith a portion of the light modulated by said device voltages corresponding' to said light V variations, and reintroducing said voltages into said electro-mechanical device after a phase reversal thereof.

5. In a soundvrecording system, the combination of means for producing a light beam, means for projecting said beam upon a film, a mirror adapted to deflect said beam with respect to said film, an armature adapted to move said mirror, a

-coil for producing a magnetic field in which said armature is located, an amplifier connected to said coil for impressing currents of different amplitudes and frequencies on said coil, means for translating light variations caused by said mirror complish the same overall correction as where the galvanometer is particularly constructed for generating the feedback voltages.

I claim: f 1. In a resonance correcting system for an electro-mechanical device, the combination of a movmovements into corresponding voltages, a second amplifier for amplifying said voltages, and means for connecting the output of said second amplifier to the input ci said first-mentioned amplier.

6. A sound recording system in accordance with claim A5 in which aphase and frequency characteristic adjusting network is connected between the output of said second amplifier and the input to said first-mentioned amplifier.

KURT SINGER-'1%. 

